PROJECT SUMMARY/ABSTRACT The adaptive response of cells to increased unfolded proteins within the endoplasmic reticulum, or the unfolded protein response (UPR), is a fundamental and conserved cellular survival mechanism. Cancer development is often associated with a range of cytotoxic conditions like hypoxia, nutrient deprivation, and pH changes. These conditions trigger a set of cellular stress response pathways including the ER-stress response. Many aspects of the endoplasmic reticulum (ER)-stress response are adaptive and protect tumor cells from cell death suggesting a crucial role in tumor growth. Much less is known about the role of ER-stress response pathways in early stages of cancer and tumor progression. Activating mutations in Ras proto-oncogenes are among the most common in human cancer. Mouse and human keratinocytes and primary lung epithelial cells respond to expression of oncogenic Ras with proliferation followed by premature senescence. Escape from the senescence response is directly linked to malignant progression. IRE1? is an ER transmembrane protein that has both serine/threonine kinase and endoribonuclease domains. In response to ER-stress IRE1? splices the XBP1 transcript to generate a potent transcription factor that induces expression of genes critical for resolution of ER stress, and in some studies is important in expression of genes regulating the malignant phenotype. The IRE1? nuclease also targets other mRNAs for degradation in a process called RIDD (regulated IRE1?- dependent decay), which can function in the UPR to reduce levels of mRNA?s that encode proteins processed through the ER but also mediates cell death to unremitting ER stress. Both IRE1? and spliced Xbp1 are elevated in cancer, and specific somatic mutations within the IRE1? kinase and RNase domains have been identified in cancers but the function consequences of these are unknown. The major goal of this proposal is to understand how IRE1? expression and RNase outputs modulate cellular responses to oncogenic Ras and promote or suppress Ras-driven cancer. Based on preliminary and submitted data we hypothesize that IRE1? is a conditional tumor suppressor whose actions in cancer depend on the balance between Xbp1 splicing and RIDD. We propose that both ER stress levels and Ras signal strength regulate the balance of IRE1? outputs driving the RNase towards Xbp1 splicing and proliferation or RIDD and senescence. Further we propose a new paradigm of ER stress dependent and independent RIDD targets. Id1 mRNA is a major ER stress independent, Ras dependent RIDD target whose degradation by the IRE1? RNase is essential for oncogene induced senescence. We will use mouse models of KRas-induced NSCLC and epidermal squamous cancer to test the effect of Ire1? or Xbp1 ablation, IRE1? overexpression and specific IRE1? human cancer mutations on cancer development. Together these studies will provide new and impactful information on the role of the IRE1? pathway in epithelial cell carcinogenesis and determine if IRE1? represents a significant target for anticancer therapeutics.